The present invention relates to an anode in which an anode active material layer containing silicon (Si) as an element is provided on an anode current collector and a battery including it.
In recent years, as mobile devices have been highly advanced and multi-functionalized, high capacity of secondary batteries, the power source thereof has been demanded. As a secondary battery to meet such a demand, there is a lithium ion secondary battery. However, the lithium ion secondary battery currently in practical use uses graphite for the anode, and therefore the battery capacity is in a saturated state, and it is difficult to attain significant high capacity. Therefore, it has been considered to use silicon or the like for the anode, and it has been reported that an anode active material layer is formed on an anode current collector by vapor-phase deposition method or the like recently. Since expansion and shrinkage due to charge and discharge are large when silicon or the like is used, lowered cycle characteristics due to pulverization has been an issue. However, by using vapor-phase deposition method or the like, pulverization can be inhibited, and the anode current collector and the anode active material layer can be integrated. Therefore, electron conductivity in the anode becomes very favorable, and improvement in both the capacity and the cycle life is promising.
However, even in the anode in which the anode current collector and the anode active material layer are integrated, there has been a shortcoming that when charge and discharge are repeated, the anode active material layer is strongly expanded and shrunk, a stress is applied between the anode current collector and the anode active material layer, fall off or the like of the anode active material layer is caused, leading to lowered cycle characteristics. Therefore, it has been considered that by roughening the anode current collector, contact characteristics with the anode current collector are improved (for example, refer to International Publication No. WO01/031723 and Japanese Unexamined Patent Application Publication No. 2002-313319). In these publicly known examples, an anode current collector in which a projection is formed in an electrolytic copper foil having concavity and convexity by electrolytic plating is described.
However, there has been a shortcoming as follows. That is, the electrolytic copper foil having concavity and convexity as above has a structure in which columnar crystal is oriented in the thickness direction, and the grain boundary often exists in the concavity portion. Therefore, when the anode active material layer is expanded and shrunk due to charge and discharge, the anode is often broken along the grain boundary of the concavity portion. Further, there has been a shortcoming that since the surface roughness of the electrolytic copper foil corresponds to the grain diameter of columnar crystal, the thickness of the electrolytic copper foil largely affects the surface roughness, and it is difficult to change the surface roughness regardless of the thickness. Further, in such an electrolytic copper foil, the surface roughness often becomes larger than the targeted value, and further it is difficult to adjust the surface roughness of the both faces to the same degree.